This invention relates to a method for two-stage classification of a granular material, which is separated in a first classifier into a first oversized material and a first undersized material, the latter of which is separated in a second classifier into a second oversized material and a second undersized material. The invention further relates to a device for the performance of the method.
German patent document DE-A-195 26 040 issued Jan. 23, 1997 to L. Kimmeyer for a Method and Device for Commination of Ground Stock describes a two-stage classification of a granular material, namely a cement grinding feed generated in a mill, which is classified in a first classifier into a first oversized material and a first undersized material. The first undersized material is then classified in a second classifier into a second oversized material and a second undersized material, that is, the finished fine cement. The oversized material from the first classifier is recycled in closed circuit to the mill, which can be a roller mill with autogenous size reduction, a roll mill, a ring mill, or another size-reduction machine. The second oversized material from the second classifier is recycled to the material inlet of the first classifier. The second classifier receives at most, in any case, only that quantity of undersized material from the first classifier that is left as undersized material, according to the particle-size distribution of the fresh feed material and the setting of the separation limit of the first classifier. It is thus possible for the second classifier to experience operating conditions in which the second classifier receives too small a quantity of undersized material as feed material from the first classifier, so that the feed loading of the second classifier declines to the extent that an excessively high sharpness of separation comes about in the second classifier, resulting in a steep gradation curve or a narrow particle-size distribution, which would not be desired, at least for the product cement.
European patent EP 0650763B1 issued Oct. 6, 1994 to H. Sussegger for a Closed Circuit Grinding Plant discloses two classifiers connected in series, in which a classifying air stream laden with an intermediate/undersize fraction from the first classifier is inlet to a second classifier for the purpose of further classification. The problem of unsuitable feed loading and thus of the unsuitable sharpness of separation in the second classifier does not arise, because a separate mill is assigned to the second classifier, the discharge material of which mill is likewise inlet to the second classifier.
It is an object of the invention to develop the two-stage classification of a granular material, in particular of a cement grinding feed, in two classifiers connected in series, in such a way that the separative behavior or the separative action and thus the particle-size distribution of the undersized material coming from the second classifier, which is normally the finished material, can be modified with simple resources.
Characteristic for the two-stage classification of a granular material according to the invention is that the oversized material from the second classifier is not, as is otherwise common, recirculated to the material inlet of the first classifier and/or to the mill, but part or all of the oversized material from the second classifier is again fed to the second classifier in closed circuit. Accordingly, the feed loading and thus the separative behavior of the second classifier are set via the quantity of oversized material recirculated to the second classifier in such a way that a second undersized material, as a rule the finished product having the desired particle-size distribution, can be withdrawn therefrom. Thus, in order to broaden the particle-size distribution of the second undersized material (finished material) or to make the gradation curve flatter, the quantity of recirculated oversized material from the second classifier is increased. Furthermore, the recirculation of at least a substream of the second oversized material from the second classifier back into this classifier increases the residence time of this material in the second classifier, which has a beneficial effect for the drying of moist classifier feed when hot gas is employed as classifying air.
The above-described controllable feed loading of the second classifier according to the invention can also be automated by a control action on the quantity of oversized material recirculated to the second classifier in dependence on the steepness of the gradation curve of the finished material or the feed loading of the second classifier.